Steel alloy, a holder or a holder detail for a plastic moulding tool, a tough hardened blank for a holder or holder detail, a process for producing a steel alloy

ABSTRACT

A steel alloy is described that, in weight-%, includes 0.08-0.19 C, where 0.16≦C+N≦0.28, 0.1-1.5 Si, 0.1-2.0 Mn, 13.0-15.4 Cr, 0.01-1.8 Ni, 0.01-1.3 Mo, max. 0.7 V, max. 0.25 S, max. 0.01 Ca, max. 0.01 O, in order to improve the machinability of the steel, and balance iron and unavoidable impurities, wherein the steel alloy has a microstructure which, in a tough hardened condition, comprises a martensitic matrix with up to 30 vol-% ferrite.

TECHNICAL FIELD

The invention relates to a steel alloy and particularly to a steel alloyfor the manufacturing of holders or holder details for plastic mouldingtools, plastic and rubber moulds with moderate requirement onpolishability, dies for plastic extrusion and also for constructionalparts. The invention also concerns holders and holder detailsmanufactured of the steel, as well as blanks made of the steel alloy forthe manufacturing of such holders and holder details. The invention alsoconcerns a production method of said steel alloy where improvedproduction economy may be provided.

BACKGROUND OF THE INVENTION

Holders and holder details for plastic moulding tools are employed asclamping and/or framing components for the plastic moulding tool in toolsets, in which tool the plastic product shall be manufactured throughsome kind of moulding method. Among conceivable holder details there canbe mentioned bolster plates and other construction parts as well asheavy blocks with large recesses which can accommodate and hold theactual moulding tool. A steel which is manufactured and marketed by theapplicant under the registered trade name RAMAX S® has the followingnominal composition in weight-%: 0.33 C, 0.35 Si, 1.35 Mn, 16.6 Cr, 0.55Ni, 0.12 N, 0.12 S, balance iron and impurities from the manufacturingof the steel. The closest comparable standardized steel is AISI 420F.Steels of this type have an adequate corrosion resistance, and arehardened and tempered to have a martensitic microstructure.

In recent years several steels have been developed which seeks toimprove the features of steels for this field of application.Particularly, the corrosion resistance, ductility, hardenability andmachinability are properties which have gained extensive focus in orderto improve the features of the steels. These steels contain loweramounts of carbon and chromium than the above steels. Furthermore,copper is added and the amount of silicon, manganese and nickel aremodified. In order to obtain very low carbon contents, the melt has tobe processed in an additional process step. This so calleddecarburization requires a converter which is equipped with means forblowing gas, normally oxygen or a mixture of oxygen and argon throughthe melt. This extra process step results in higher production costs.

An example of a steel alloy for use in the manufacture of plasticinjection mold base components is disclosed in U.S. Pat. No. 6,358,334.The steel alloy comprises 0.03-0.06% C, 1.0-1.6% Mn, 0.01-0.03% P,0.06-0.3% S, 0.25-1.0% Si, 12.0-14.0% Cr, 0.5-1.3% Cu, 0.01-0.1% V,0.02-0.08% N, the rest Fe with trace amounts of ordinarily presentelements. Compared to an AISI 420F type of steel, the steel is said tohave a beneficial combination of features due to reduced hardness andhardenability, improved ductility, corrosion resistance, hot strengthand weldability as well as improved surface quality in hot workedcondition.

US 2002/0162614 discloses a maraging steel alloy suitable for themanufacture of a frame construction for plastic moulds, a mould part anda process for production of the steel alloy which is said to obtain animproved machinability, good weldability and high corrosion resistance.The alloy comprises 0.02-0.075% C, 0.1-0.6% Si, 0.5-0.25% S, up to max.0.04% P, 12.4-15.2% Cr, 0.05-1.0% Mo, 0.2-1.8% Ni, up to max. 0.15% V,0.1-0.45% Cu, up to max. 0.03% Al, 0.02-0.08% N and residual Fe andimpurities from the manufacturing.

WO 2006/016043 discloses a martensitic stainless steel for a mould or amould part for plastic injection moulding. The steel alloy comprises0.02-0.09% C, 0.025-0.12% N, max. 0.34% Si, max. 0.080% Al, 0.55-1.8%Mn, 11.5-16% Cr, and possibly up to 0.48% Cu, up to 0.90% (Mo+W/2), upto 0.90% Ni, up to 0.090% V, up to 0.090% Nb, up to 0.025% Ti, possiblyup to 0.25% S, the rest Fe and impurities from the manufacturing. Thesteel is said to obtain an improved weldability, good corrosionresistance, good thermal conductivity and small problems during forgingand recycling when compared for example to the steel disclosed in U.S.Pat. No. 6,358,334.

A steel which is manufactured and marketed by the applicant under theregistered trade name RAMAX 20 belongs to the recently developed steels.The steel alloy has the following nominal composition: 0.12% C, 0.20 Si,0.30 Mn, 0.10 S, 13.4 Cr, 1.60 Ni, 0.50 Mo, 0.20 V, and 0.105 N, therest Fe and impurities from the manufacturing. The manufacturing of thesteel can be performed without any need of a subsequent decarburizationstep. The steel has excellent machinability, good corrosion resistanceand hardenability, uniform hardness in all dimensions and goodindentation resistance which result in lower mould production andmaintenance costs and is a successful product on the market.

The above mentioned steels have become significantly more expensive tomanufacture because the cost of certain alloying elements has increasedlately. Furthermore, the low carbon content in some of these steelsmakes it necessary to perform a decarburization of the melt whichresults in increased production costs. Therefore it exists a demand fora steel which may be produced at lower alloying costs without anysignificant reduction in respect of the most important features of asteel for this application, e.g. corrosion resistance, hardenability,machinability and hardness and which can be manufactured without anyneed of a subsequent decarburization step.

DISCLOSURE OF THE INVENTION

It is an object of the invention to provide a steel alloy andparticularly a steel alloy for the manufacturing of holders and holderdetails for plastic moulding tools, plastic and rubber moulds withmoderate requirement on polishability, dies for plastic extrusion andalso for constructional parts which can be manufactured at loweralloying costs. This can be achieved with a steel alloy which ischaracterized in that it has a chemical composition which contains inweight-%:

-   -   0.08-0.19 C    -   0.16≦C+N≦0.28    -   0.1-1.5 Si    -   0.1-2.0 Mn    -   13.0-15.4 Cr    -   0.01-1.8 Ni    -   0.01-1.3 Mo    -   optionally vanadium up to max. 0.7 V,    -   optionally S in amounts up to max. 0.25 S and optionally also    -   Ca and O in amounts up to    -   max. 0.01 (100 ppm) Ca,    -   max. 0.01 (100 ppm) O, in order to improve the machinability of        the steel,    -   balance iron and unavoidable impurities, and has a        microstructure which in tough hardened condition comprises a        martensitic matrix containing up to 30 vol-% ferrite, and having        a hardness in its tough hardened condition between 290-352 HB.

The invention also aims to provide a steel alloy with an improvedmachinability since a large part of the manufacturing cost relates tothis operation, which is performed by different cutting operations. Itis also preferred that the steel alloy of this invention fulfils thefollowing requirements:

-   -   an adequate corrosion resistance,    -   a hardness of 290-352 HB in tough hardened condition which gives        the steel a beneficial combination of hardness and        machinability,    -   an adequate hardenability, considering the steel shall be        possible to be used for the manufacturing of holder blocks made        of plates which may have a thickness of up to at least 300 mm        and in some cases even up to 400 mm thickness.    -   an adequate ductility/toughness,    -   a adequate polishability, at least according to a preferred        embodiment, in order to able to be used also for moulding tools        on which moderate demands are raised as far as polishability is        concerned,    -   a adequate hot ductility in order avoid extensive machining for        removal of defects formed during the hot working operation.

The invention also concerns blanks made of the steel alloy for themanufacturing of such holders and holder details. It is a further objectof this invention to provide a production method with improvedproduction economy.

According to the broadest aspect of this invention, the steel alloy forthe manufacturing of holders or holder details for plastic mouldingtools, plastic and rubber moulds, dies for plastic extrusion andconstructional parts of holders and holder details shall have a chemicalcomposition which contains (in weight-%) 0.08-0.19 C, 0.16<C+N<0.28,0.1-1.5 Si, 0.1-2.0 Mn, 13.0-15.4 Cr, 0.01-1.8 Ni, 0.01-1.3 Mo, max. 0.7V, max. 0.25 S, max. 0.01 (100 ppm) Ca and max. 0.01 (100 ppm) O,balance iron and unavoidable impurities and containing up to 30 vol-%ferrite in its matrix.

According to a second aspect of the invention an improvement inmachinability and further reduction in alloying costs can be obtained ifthe steel contains (in weight-%) 0.10-0.15 C, 0.08<N≦0.14N, where0.17<C+N<0.25, 0.7-1.2 Si, 0.85-1.8 Mn, 13.5-14.8 Cr, 0.10-0.40 Mo,0.1-0.55 Ni, 0.09<V≦0.20, the balance being iron and unavoidableimpurities, and containing up to 15 vol-% ferrite in its matrix.Preferably, the chemical composition of the steel contains (in weight-%)0.10-0.15 C, 0.08<N≦0.14 N, where 0.17<C+N<0.25, 0.75-1.05 Si, 1.35-1.55Mn, 13.6-14.1 Cr, 0.15-0.25 Mo, 0.30-0.45 Ni, 0.09<V≦0.15, the balancebeing iron and unavoidable impurities, and containing up to 10 vol-%ferrite in its matrix.

In a variant of the steel, the performed tests have shown that anunexpected improvement in machinability at the same time as a reductionof alloying and production costs can be obtained if the steel alloy hasa chemical composition which contains (in weight-%) 0.08-0.19 C,0.16<C+N<0.28, 0.75-1.05 Si, 1.05-1.8 Mn, 13.0-15.4 Cr, 0.15-0.25 Ni,0.15-0.55 Mo, max. 0.7 V, max. 0.25 S, max. 0.01 (100 ppm) Ca and max.0.01 (100 ppm) O, balance iron and unavoidable impurities and containingup to 10 vol-% ferrite in its matrix.

As far as the importance of the separate elements and their interactionin the steel are concerned, the following may be considered to applywithout binding the claimed patent protection to any specific theory. Inthis text, always weight-% is referred to when amounts of alloyingelements are concerned and volume-% is referred to when the structuralcomposition of the steel is concerned, e.g. carbides, nitrides,carbonitrides, martensite or ferrite, if not otherwise is stated. Inthis text, M(C,N)-carbides, M₂₃C₆-carbides, M₇C₃-carbides etc. refer tocarbides and nitrides as well as carbonitrides, if not otherwise stated.

Carbon and nitrogen are elements which have a great importance for thehardness and ductility of the steel. Carbon is also an importanthardenability promoting element. Carbon, however, binds chromium in theform of chromium carbides (M₇C₃-carbides) and may therefore impair thecorrosion resistance of the steel. The steel therefore may contain max0.19% carbon, preferably max 0.15% carbon and even more preferred max0.14% carbon. However, carbon also exists together with nitrogen as adissolved element in the tempered martensite in order to contribute tothe hardness thereof, and acts as an austenite stabilizer. The minimumamount of carbon in the steel shall be 0.08%, preferably more than0.09%. In a preferred embodiment, the carbon content is at least 0.10%.Nominally the steel contains 0.12% C.

Nitrogen contributes to the provision of a more even, more homogenousdistribution of carbides and carbonitrides by affecting thesolidification conditions in the alloy system such that largeraggregates of carbides are avoided or are reduced during thesolidification. The proportion of chromium rich M₂₃C₆-carbides also isreduced in favour of smaller M(C,N), i.e. vanadium-carbides, which has afavourable impact on the ductility/toughness and the corrosionresistance. Nitrogen contributes to the provision of a more favourablesolidification process implying smaller carbides and nitrides, which canbe broken up during the working to a more finely dispersed phase. Thesecarbides will also contribute to finer grain size of the steel. Nitrogenalso acts as an austenite stabilizer.

From these reasons nitrogen shall exist in an amount of at least 0.05%,preferably more than 0.08%, but not more than 0.20%, preferably max0.13%, and even more preferred max 0.11%. Nominally the steel contains0.09% N. At the same time the total amount of carbon and nitrogen shallsatisfy the condition 0.16≦C+N≦0.28, preferably 0.17≦C+N≦0.25. In apreferred embodiment, the sum of carbon and nitrogen shall be at least0.19% but suitably max 0.23%. Nominally, the steel contains 0.21% (C+N).In the hardened and tempered condition of the steel, nitrogen issubstantially dissolved in the martensite in the form ofnitrogen-martensite in solid solution and thence contributes to thedesired hardness.

In summary, as far as the content of nitrogen is concerned, it can bestated that nitrogen shall exist in the said minimum amount in order tocontribute to the desired corrosion resistance by increasing the socalled PRE-value of the matrix of the steel, to exist as a dissolvedelement in the tempered martensite which contributes to the hardness ofthe martensite and to form carbonitrides, M(C, N), to a desired degreetogether with carbon, but not exceed said maximum content, maximizingthe content of carbon+nitrogen, where carbon is the most importanthardness contributor.

Silicon increases the carbon activity of the steel and thence thetendency to precipitate more primary carbides. Also, a positive effectmay be obtained in the steels ability to reduce adhesive wear andgalling on the cutting tools, and chip breaking properties can beimproved by silicon. Moreover, silicon is a ferrite stabilizing elementand it shall be balanced in relation to the ferrite stabilizing elementschromium and molybdenum in order for the steel to obtain a desiredferrite content of up to 30%, thereby providing the steel desiredmachinability and hot ductility. For the inventive steel however, itappears as if silicon contributes to the improvement in machinabilitynot only by its ferrite promoting feature. At the same time, the steelcontains a lower content of carbon than is conventional in steels forthe application in question but a higher content than has been suggestedin some of the recently developed steels mentioned above. The steeltherefore shall contain at least 0.1% Si, preferably more than 0.6%, andeven more preferred at least 0.7% Si. Generally the rule shall applythat the ferrite stabilizing elements shall be adapted to the austenitestabilizing ones in order to obtain the desired formation of ferrite inthe steel. The maximum content of silicon is 1.5%, preferably max 1.2%.A preferred silicon content is 0.75-1.05%. Nominally the steel contains0.90% silicon.

Manganese is an element which promotes hardenability, which is afavourable effect of manganese and can also be employed for sulphurrefining by forming manganese sulphides in the steel which also promotesthe machinability. In a preferred embodiment, the inventive steel shallhave a hardenability which makes bars at larger dimensions possible tobe hardened by cooling in air, thereby eliminating the need ofsubsequent flattening of the hardened bars. Manganese therefore shallexist in a minimum amount of 0.1%, preferably at least 0.85% and evenmore preferred at least 1.05%. Manganese, however has a segregationtendency together with phosphorous which can give rise totempering-embrittlement wherefore the content of phosphorous shall becontrolled to not exceed impurity level. Manganese also is an austenitestabilizing element. Manganese therefore must not exist in an amountexceeding 2.0%, preferably max. 1.8% and even more preferred max. 1.6%.In a preferred embodiment, the manganese content is 1.35-1.55% and evenmore preferred 1.40-1.45%. Nominally the steel contains 1.45% Mn.

Chromium is an important alloying element and is essentially responsiblefor provision of the stainless character of the steel, which is animportant feature of holders and holder details for plastic mouldingtools, as well as for the plastic moulding tool itself, which often isused in damp environments, which may cause less corrosion resistantsteels to rust.

Chromium also is the most important hardenability promoting element ofthe steel. However, no substantial amounts of chromium are bound in theform of carbides, because the steel has comparatively low carboncontent, wherefore the steel can have a chromium content as low as 13.0%and nevertheless get a desired corrosion resistance. Preferably thesteel, however, contains at least 13.5%. The upper limit is determinedin the first place by cost reasons, reduced hardness due to carbideprecipitation, and the risk for chromium segregations. The steeltherefore must not contain more than max. 15.4% Cr, preferably max.14.8% Cr, and even more preferred max 14.5% Cr. Chromium is a ferritestabilizer and, if present in amounts within the upper ranges of thedefined interval, it may preferably be combined with a high carboncontent, typically 0.14-0.18% C. However, according to a preferredembodiment, the chromium content is kept at more moderate amounts,typically 13.6-14.1%. Nominally, the steel contains 13.9% Cr.

Nickel is an element which improves the toughness of the steel. Furtherit is beneficial for hardenability. Therefore, nickel shall exist in thesteel in a minimum amount of 0.01%, preferably at least 0.15%. For costreasons and because nickel acts as an austenite stabiliser, the contentshould be limited to max. 1.8%, preferably to max. 1.5%.

In order to further reduce the costs of alloying elements, the nickelcontent may be reduced even further, to an interval of 0.15-0.55%,preferably 0.20-0.50% and even more preferred 0.30-0.45% Ni. In orderfor this embodiment to obtain the desired hardenability, the low nickelcontent is combined with a manganese content of 1.05-1.8% Mn, preferably1.35-1.55% Mn, possibly also with a silicon content of 0.75-1.05% Si.Nominally the steel contains 0.36 Ni.

In a variant of the steel, the steel does not contain any intentionallyadded vanadium. However, in a preferred embodiment, the steel of theinvention also contains an active content of vanadium in order to bringabout a secondary hardening through precipitation of secondary carbidesin connection with the tempering operation, wherein the temperingresistance is increased. Vanadium, when present, also acts as a graingrowth inhibitor through the precipitation of M(N,C)-carbides which isbeneficial. If the content of vanadium is too high, however, there willbe formed large primary M(N,C)-carbides during the solidification of thesteel, which will not be dissolved during the hardening procedure. Forthe achievement of the desired secondary hardening and to avoid graingrowth, the vanadium content shall be at least 0.05% V, preferably 0.07%V and even more preferred more than 0.09% V. The upper amount ofvanadium is determined primarily to avoid the formation of large,undissolvable primary carbides in the steel and for that reason thecontent of vanadium should be max. 0.7% V, preferably max 0.25% V andeven more preferred max. 0.20% V, but may be reduced even further tomax. 0.15% V. The nominal content is 0.10% V.

Preferably, the steel also contains an active content of molybdenum,e.g. at least 0.05%, preferably at least 0.10%, in order to give ahardenability promoting effect. Molybdenum also promotes the corrosionresistance. From cost reasons though, it is desirable to minimisemolybdenum, but still both corrosion resistance and hardenability shallbe sufficient.

When tempering, molybdenum also contributes to increasing the temperingresistance of the steel, which is favourable. On the other hand, a toohigh content of molybdenum may give rise to an unfavourable carbidestructure by causing a tendency to precipitation of grain boundarycarbides and segregations and for that reason the maximum amount ofmolybdenum is set to 1.3%. In summary, the steel shall contain abalanced content of molybdenum in order to take advantage of itsfavourable effects but at the same time avoid those ones which areunfavourable. A suitable molybdenum content is between 0.10-0.40%. In apreferred embodiment the molybdenum is 0.15-0.25% Mo. Nominally, thesteel contains 0.20% Mo.

Normally, the steel does not contain tungsten in amounts exceeding theimpurity level, but may possibly be tolerated in amounts up to 1%.

Copper promotes the corrosion resistance and hardness of the steel andwould for that reason be a suitable alloying element in the steel.However, copper impair the hot ductility even in low amounts and it isimpossible to extract copper from steel once it's added. This factcontributes drastically to impair the possibility to internallyrecycling the steel in the mill. Logistic scrap handling must in suchcases be built up to also avoid raised Cu contents in grades nottolerant to high Cu contents. This is well documented for e.g. hot worktool steels where ductility at ambient or elevated temperatures at theusing in a specific application are negatively affected (ref. to Ernstet al. Impact of scrap use on the properties of hot-work tool steels,European Commission technical steel research, EUR20906, 2003). For thatreason, copper shall be tolerated only as unavoidable andunintentionally added element from the scrap. The maximum amount ofcupper in the steel is 0.40%, preferably 0.25% and even more preferredmax. 0.15% Cu.

Normally, strong carbide forming alloying elements such as titanium andniobium are also undesired in the steel of the invention since theywould impair the toughness and ductility.

The steel of the invention shall be possible to be delivered in itstough-hardened condition, which makes it possible to manufacture largesized holders and mould tools through machining operations. Despite thefact that the hardenability promoting elements nickel and molybdenum arereduced, the steel possesses a hardenability which allows hardening bycooling in air, even of bars with very large dimensions. By cooling inair, distortion and high stresses can be avoided in the steel, which canbe released during mould manufacturing. The hardening is carried outthrough austenitizing at a temperature of 900-1100° C., preferably at950-1025° C., or at about 1000° C., followed by cooling in oil or in apolymer bath, by cooling in gas in a vacuum furnace, or most preferredin air. The high temperature tempering for the achievement of a toughhardened material with a hardness of 290-352 HB which is suitable formachining operations, is performed at a temperature of 510-650° C.,preferably at 540-620° C., for at least one hour, preferably throughdouble tempering; twice for two hours.

The steel may, according to a preferred embodiment, also contain anactive content of sulphur, possibly in combination with calcium andoxygen, in order to improve the machinability of the steel in its toughhardened condition. In order to obtain further improvement in terms ofmachinability, the steel should contain at least 0.10% S if the steeldoes not also contain an intentionally added amount of calcium andoxygen. The maximum sulphur content of the steel is 0.25%, preferablymax 0.15%, when the steel is intentionally alloyed with a content ofsulphur. A suitable sulphur content in this case may be 0.13%. Also anon-sulphurized variant of the steel can be conceived. This variant willobtain a better polishability. In this case the steel does not containsulphur above impurity level, and nor does the steel contain any activecontents of calcium and/or oxygen.

It is thus conceivable that the steel may contain 0.035-0.25% S incombination with 3-100 ppm Ca, preferably 5-75 ppm Ca, suitably max. 40ppm Ca, and 10-100 ppm 0, wherein said calcium, which may be supplied assilicon-calcium, CaSi, in order to globulize existing sulphides to formcalcium sulphides, counteracts that the sulphides get a non-desired,elongated shape, which might impair the ductility.

According to the broadest aspect of this invention, an improvement inmachinability in hardened and tempered condition can be achieved if thesteel contains up to 30 vol-% ferrite. The performed tests have alsoshown that the inventive steel meets the requirements set for itsintended use. Further, the steel is possible to produce at loweralloying and production costs.

The performed tests have also revealed, very surprisingly, that in avariant of the steel an improved machinability can be obtained even atvery low levels, i.e. up to about 10%. In this variant of the steel, thesilicon content is 0.75-1.05%. Particularly molybdenum, which has becomeexpensive, is kept at low levels, and a preferred molybdenum content is0.15-0.25%. Also nickel has become expensive and shall therefore be keptat low levels. A suitable nickel content is 0.15-0.55%, preferably0.30-0.45%, which preferably is combined with a manganese content of1.05-1.8% Mn, preferably 1.35-1.55% Mn, in order to obtain the desiredhardenability of the steel. Nominally the steel contains 0.36 Ni, 1.45Mn and 0.90 Si. In order to further reduce the alloying costs it ispossible to reduce the vanadium content to 0.10-0.15% and still obtainan effect as grain growth inhibitor and adequate ductility/toughness.

Further characteristics, aspects and features of the steel according tothe invention, and its usefulness for the manufacturing of holders andmoulding tools, will be explained more in detail in the followingthrough a description of performed experiments and achieved results.

BRIEF DESCRIPTION OF DRAWINGS

In the following description of performed experiments and achievedresults according to the new variant of the steel, reference will bemade to the accompanying drawings, in which

FIG. 1 shows a holder block of a typical design, which can bemanufactured of the steel according to the invention,

FIG. 2A is a chart showing the hardness of a first set of steels,produced in the form of so called Q-ingots (50 kg laboratory heats),after hardening but before tempering, versus the austenitizingtemperature at a holding time of 30 min,

FIG. 2B shows corresponding graphs for another number of tested steelsmanufactured as Q-ingots,

FIG. 2C shows corresponding graphs for yet another number of testedsteels manufactured as Q-ingots,

FIG. 2D shows a corresponding graph for a tested steel produced at 60tons production scale (so called DV-heat),

FIG. 3 shows tempering curves for those steels which have been hardenedfrom 1000° C.,

FIG. 4A-B is a chart which showing hardenability curves for the steels,

FIG. 5A-D are bar charts illustrating results from machinability testingof steels, manufactured at laboratory scale and production scale,

FIGS. 6A, B is a chart which shows the hot ductility for a number ofsteels,

FIG. 7 is a photo showing the microstructure for a preferred embodimentof the new variant of the steel, and

FIG. 8 shows polarisation curves for the inventive steel and somereference steels.

EXAMINATION OF STEELS

FIG. 1 shows a holder block 1 of a typical design, which shall bepossible to be manufactured of the steel according to the invention. Inthe block 1 there is a cavity 2, which shall accommodate a mould tool,usually a plastic moulding tool. The block 1 has considerable dimensionsand the cavity 2 is large and deep. Therefore, a number of differentrequirements are raised on the material according to the invention, i.e.an adequate hardenability with reference to the considerable thicknessof the block, and a good ability to be machined by means of cuttingtools, such as mill cutters and borers.

Material

Test materials were manufactured both at laboratory scale and productionscale. Initially, three rounds with tests on so called Q-ingots (50 kglaboratory heats) were performed (Q9261-Q9284) followed by one round oftests on materials manufactured at production scale (inventive steel No.4). Thereafter, a new set of Q-ingots were manufactured (Q9294-Q9295)and finally a round of tests were performed on materials manufactured atfull production scale (inventive steel No. 5).

The compositions of the Q-ingots are shown in Table VI where ingot Q9261is a reference composition in accordance to reference material No. 1 andQ9271 and Q9283 are reference materials where Q9283 contains a higheramount of S. The Q-ingots were forged to the shape of bars of size 60×40mm, whereupon the rods were cooled in air to room temperature. The rodswere heated to 740° C., cooled at a cooling rate of 15° C./h to 550° C.,there from free cooling in air to room temperature.

The compositions of the steels manufactured at production scale areshown in table VIII below. Commercial steels (steels No 1, 2 and 3) forcomparison of the features of the inventive steels No. 4 and 5 wereobtained from the commercial market and no heat treatment or othertreatment was performed to them.

The inventive steel No. 4 was manufactured as a 6 tons full scale testheat and ingots were cast which were manufactured to test pieces byeither hot rolling or forging at a temperature of 1240° C. The testpieces were cooled to an isothermal annealing temperature of 650° C. andwere subjected to an isothermal annealing at the isothermal annealingtemperature during 10 h, thereafter cooled in free air to roomtemperature. The test pieces were then hardened by austenitizing at atemperature of 1000° C., 30 min, and tempered twice during two hours ata temperature of 550-620° C.

The inventive steel No 5 was manufactured as a 60 tons full scale testheat was produced in a conventional metallurgical process using anelectric arc furnace, processed in a secondary ladle step and cast intoingots. The ingots were forged at a temperature of 1240° C. to the shapeof bars of size 610×254 mm, 600×100 mm and 610×305 mm respectively. Thebars were cooled to an isothermal annealing temperature of 650° C. andwere subjected to an isothermal annealing at the isothermal annealingtemperature during 10 h, thereafter cooled in free air to roomtemperature. The bars were then hardened by austenitizing at atemperature of 1000° C., 30 min, and tempered twice during two hours ata temperature of 550-620° C.

TABLE VI Test materials manufactured at laboratory scale; chemicalcomposition in weight-%, balance Fe and unavoidable impurities Q-ingotNo. C Si Mn S Cr Ni Mo V N Q9261 = 0.15 0.09 0.89 0.14 12.9 1.69 0.550.22 0.12 ref Q9262 0.13 0.24 1.10 0.14 13.0 0.84 0.21 0.15 0.10 Q92630.13 0.24 1.07 0.13 12.9 0.84 0.21 0.15 0.10 Q9264 0.12 0.26 1.11 0.1413.0 0.84 0.11 0.14 0.07 Q9271 = 0.14 0.12 0.90 0.10 13.2 1.65 0.52 0.240.08 ref Q9272 0.15 0.93 0.90 0.13 14.5 0.96 0.22 0.33 0.08 Q9273 0.130.93 0.84 0.12 13.5 0.08 0.21 0.21 0.08 Q9274 0.15 0.75 0.78 0.13 14.70.07 0.20 0.20 0.10 Q9275 0.12 0.79 0.90 0.13 15.8 0.95 0.21 0.20 0.06Q9276 0.07 0.78 0.90 0.11 14.4 0.93 0.20 0.20 0.05 Q9283 = 0.12 0.091.16 0.13 13.4 1.68 0.53 0.25 0.09 Q9271 + S Q9284 0.12 0.87 1.09 0.1214.8 0.96 0.27 0.22 0.12 Q9294 0.12 0.89 1.54 0.12 14.0 0.21 0.21 0.110.09 Q9295 0.11 0.94 1.38 0.11 14.4 0.52 0.21 0.10 0.089

TABLE VIII Steel composition of examined steels manufactured atproduction scale; chemical composition in weight-%, balance Fe andunavoidable impurities C Si Mn S Cr Ni Mo V Cu N Steel No 1 0.15 0.181.26 0.08 13.6 1.6 0.48 0.20 0.15 0.083 Steel No. 2 0.045 0.40 0.92 0.1412.8 0.44 0.15 0.049 0.26 0.039 Steel No. 3 0.046 0.43 1.30 0.14 12.70.18 0.02 0.032 0.63 0.047 Steel No. 4 0.14 0.89 1.11 0.14 14.3 0.960.19 0.15 0.10 0.071 Steel No 5 0.12 0.85 1.44 0.12 13.7 0.37 0.19 0.110.037 0.086Hardness and Ferrite Content after Heat Treatment

The hardness versus the austenitizing temperature is shown in FIG.2A-2D. It is evident from the charts of these drawings that thereference steels (Q9261, Q9271 and Q9283) have the highest hardness. Itis also evident that the hardness increases with increasingaustenitizing temperature. However, some of the tested steels of theinvention may obtain a hardness which is close to the hardness of thereference steels, but that require that a somewhat higher austenitizingtemperature is chosen, i.e. about 1000° C.

The hardness after tempering of some of the tested steels which havebeen hardened from 1000° C. is shown in FIG. 3. The conclusion can bedrawn from the tempering curves that these steels can be tempered downto 34 HRC through tempering in the temperature range 520-600° C. As isevident from the figure, the inventive steels No. Q9272, Q9273, Q9274and Q9284 can be tempered at higher temperatures than the otherinventive steels and still obtain a high hardness, which is beneficialfrom a stress relief point of view.

An appropriate hardness of the steel after tough-hardening is about31-38 HRC, (i.e. 290-352 HB). In Table VII below, the heat treatmentsare stated which provide a hardness within the interval to the differentsteels. The ferrite content has been measured by manual point counting(swe. rutnätsmetoden) after hardening and tempering.

TABLE VII Heat treatment for tough-hardening, measured ferrite, percentby volume Steel No. Heat treatment Ferrite content % Q9261 950° C. +580° C./2 × 2 h 0 Q9262 950° C. + 565° C./2 × 2 h 0 Q9263 950° C. + 570°C./2 × 2 h 0 Q9264 950° C. + 565° C./2 × 2 h 0 Q9271 950° C. + 585° C./2× 2 h 0 Q9272 950° C. + 555° C./2 × 2 h 4.5 Q9273 950° C. + 545° C./2 ×2 h 9 Q9274 950° C. + 535° C./2 × 2 h 32 Q9275 1000° C. + 540° C./2 × 2h  21 Q9276 1000° C. + 520° C./2 × 2 h  19 Q9283 950° C. + 585° C./2 × 2h 0 Q9284 1000° C. + 590° C./2 × 2 h  2.5 Q9294 1000° C. + 560° C./2 × 2h  8.5 Q9295 1000° C. + 560° C./2 × 2 h  7 Steel No. 4 1000° C. + 590°C./2 × 2 h  1.5-4  Steel No. 5 1000° C. + 560° C./2 h + 570° C./2 h0.05-6.5

Hardenability

The hardness after hardening is shown in the hardenability curves ofFIGS. 4A and 4B. The austenitizing temperatures are indicated in thefigure from which temperatures the samples have been cooled at differentrates.

From FIG. 4A, which shows the hardenability for some of the steelsmanufactured at laboratory scale, it is evident that steels No. Q9272,Q9294 and Q9295 austenitized at 1000° C. have the best hardenabilityamong the inventive steels. These steels have sufficient hardenabilityin order to be hardened by cooling in air at relatively thickdimensions. The other steels may be used for less thick dimensions. Thesteels in the figure which shows the lowest hardenability have low Nicontent. The best hardenability is obtained by commercial steel No. 1which is represented by the hardening curves for Q9283 and Q9271.

From FIG. 4B, which shows the hardenability for the steels manufacturedat production scale, it is evident that the inventive steels No 4 and No5 can obtain a high hardness after hardening which is equal to thecommercial steel No. 1, (Q9271 in FIG. 4A) and well above the commercialsteels No. 2 and No. 3.

Machinability Tests Performed at Laboratory Scale

The machinability of the inventive steels which were manufactured atlaboratory scale (Q-ingots) where examined and compared to the referencesteels Q9261, Q9271 and Q9283. The results are shown in table IX below.It shall be considered that laboratory manufactured steels may containdefects which impair the results.

By face milling with uncoated carbide inserts the time required to wearthe flank 0.5 mm were examined. The cutting data were as follows:

Machine type=SEKN 1203AFTN-M14 S25MMilling cutter=Seco R220.13-0040-12 Ø40 mm, 3 insertsCutting speed, vc=250 ml/minTooth feed, fz=0.2 mm/toothAxial depth of cut, ap=2 mmRadial depth of cut, ae=22.5 mmWear criteria=flank wear 0.5 mm

The result indicated that the inventive steels can obtain equal orbetter face milling properties as the commercial steels. Q9284 is bestamong the inventive steels and Q9294 and Q9295 are very good as well.

By drilling with high speed steel, the average number of drilled holesthat could be made before the drill were damaged, were examined. Thedrilling data were as follows:

Drill type: Wedevåg 120 uncoated HSS Ø2 mmCutting speed, vc: 26 m/minFeed rate, f: 0.04 mm/rev.Drill depth: 5 mm

The result indicated that the inventive steel can obtain better drillingproperties than the reference steels.

By end milling with high speed steel the time required to wear the flank0.15 mm were examined. The drilling data were as follows:

Milling cutter=Sandvik Coromant R216.33-05050-AK13P 1630 Ø5 mm,Cutting speed, vc=200 m/inTooth feed, fz=0.05 mm/toothAxial depth of cut, ap=2 mmRadial depth of cut, ae=5 mmWear criteria=flank wear 0.15 mm

The result indicated that the inventive steel can obtain better endmilling properties than the reference steels.

TABLE IX Result of machining tests on steels manufactured at laboratoryscale Drilling Steel Hardness (HB) Face milling with HSS End millingQ9261 350 n.a.  160* n.a. Q9262 348 n.a.  325* n.a. Q9271 340 7.5  69 9Q9272 350 5.9 345 14.9 Q9275 350 8 110 6.3 Q9276 350 1.1 455 9.6 Q9283330 10.8 178 7 Q9284 320 23.5 507 9.9 Q9294 333 20.1 495 — Q9295 33322.2 535 — *Cutting speed: 22 m/min n.a. not analysed

When both milling and drilling properties were considered, the resultsfor steels No. Q9284, Q9294 and Q9295 showed that an improvement inmachinability can be obtained with a steel according to the invention.

Machinability Tests Performed at Production Scale

The machinability of the inventive steels which were manufactured atproduction scale were examined by different machining operations andcompared to the machinability of some commercial steels.

FIG. 5A shows the result from face milling with coated carbide tools.The cutting data were as follows:

Machine type=Sajo VM 450Milling cutter=Sandvik Coromant R245-80Q27-12M, Ø80 mm, 6 insertsCutting speed, vc=250 m/minTooth feed, fz=0.2 mm/toothAxial depth of cut, ap=2 mmRadial depth of cut, ae=63 mmWear criteria=flank wear 0.5 mm

As is evident from FIG. 5A the inventive steel may obtain equal orbetter face milling properties than the commercial steels. Particularlythe inventive steels with somewhat lower hardness than the commercialsteels shows superior face milling properties.

FIG. 5B shows the results from cavity milling with coated carbide tools.The cutting data were as follows:

Milling tool: Coromant R200-028A32-12M, Ø40 mm, 1=145 mmCarbide grade: Coromant RCKT 1204 MO-PM 4030Wear criteria: VBmax 0.5 mmCutting speed, vc=VaryingTooth feed, fz=0.25 mm/toothAxial depth of cut, ap=2 mmRadial depth of cut, ae=12 mm

FIG. 5B shows that the inventive steel may obtain equal or better cavitymilling properties than the commercial steel No. 2 and 3, and that theinventive steel is superior the commercial steel No. 1.

FIG. 5C shows the result from drilling with high speed steel. It isevident from these tests that the inventive steel can obtain equal orbetter drilling properties than the commercial steels. The drilling datawere as follows:

Drill type: Wedevåg 120 uncoated HSS Ø5 mmCutting speed, vc: 26 m/minFeed rate, f: 0.15 mm/rev.Drill depth: 12.5 mm

FIG. 5D shows the result from end milling with high speed steel. It isevident from these tests that the inventive steel No. 5 can obtain muchbetter end milling properties than the commercial steels. The drillingdata were as follows:

Milling cutter: C200 uncoated HSS Ø12 mmCutting speed, vc: 70 m/minRadial depth of cut, ae=1.2 mmAxial depth of cut, ap=18 mmTooth feed, fz: 0.14 mm/toothWear criteria=flank wear 0.15 mm

To sum up, the results of the machinability tests are presented in TableX. In the Table, the results for the steels are presented by a value,1-5, where the value 5 represents a very good result and value 1represents unsatisfying result. The results of steel No. 4 in forgedcondition are shown at different hardness in accordance to FIGS. 5A-C,the hardness in forged condition being 310 HB and 327 HB respectively.

TABLE X Result of machining tests on steels manufactured at productionscale Face Cavity End Steel Hardness milling Drilling milling millingNo. 2 4 3 5 4 3 No. 3 3 — 5 3 3 No. 1, 4 3 4 2 4 No. 1, 5 2 — 2 3 No. 4,3 5 5 5 n.a. Hot rolled No. 4, Forged 3 5 5 5 n.a. No. 4, Forged 4 3 4 3n.a. No. 5, 4 4 5 5 5 n.a. = not analysed

Hot Ductility

The hot ductility of the inventive steel is shown in FIGS. 6A and 6B.The curves in the interval of 900-1150° C. shows the hot ductilityobtained for the steel on cooling from the hot work temperature of 1270°C. of the test pieces and the curves in the interval of 1150-1350° C.shows the hot ductility on heating of the test pieces. The inventivesteel has shown to have a good hot ductility, both at high and somewhatlower temperatures. The result shows that the inventive steels can behot worked at high temperatures and also that it can be hot worked downto 900° C. which makes it possible to hot work in one step, withoutreheating.

Microstructure

The microstructure in tough hardened condition of Steel No. 5 is shownin FIG. 7. The microstructure consists of a matrix of martensite 3.Further, the matrix contains approximately 3% ferrite 1 and somemanganese sulphides 2, MnS, can be seen. The tough hardening wasperformed at an austenitizing temperature of 1000° C., 30 min andtempering at 560° C./2 h+570° C./2 h. The manufacturing process includedforging and cooling in air. The test piece had a dimension of 610×254 mmobtained by hot rolling.

Corrosion Tests

Polarization curves were established for the steels given in Table XI interms of critical current density, Icr, for the evaluation of thecorrosion resistance of the steels. As far as this method of measurementis concerned, the rule is that the lower Icr the better the corrosionresistance.

TABLE XI Heat treatment of polarization test specimens. Cooling invacuum furnace Q-ingot Hardness Icr No. Heat treatment (HRC) (mA/cm2)Q9261 950° C. + 580° C./2 × 2 h 34.6 3.49 Q9262 950° C. + 565° C./2 × 2h 35.8 7.23 Q9263 950° C. + 570° C./2 × 2 h 34.5 6.84 Q9264 950° C. +565° C./2 × 2 h 34.3 7.90 Q9271 950 + 585/2 × 2 h 35.9 1.70 Q9272 950 +555/2 × 2 h 36.7 5.40 Q9273 950 + 545/2 × 2 h 36.5 6.28 Q9274 950 +545/2 × 2 h 31.9 4.29 Q9275 1000 + 540/2 × 2 h  34.2 4.76 Q9276 1000 +520/2 × 2 h  35.7 2.53 Q9283 950 + 585/2 × 2 h 34.3 3.08

The results showed that the steels Q9274, Q9275 and Q9276 had bettercorrosion resistance than most of the other tested steels, and thatQ9276 had best corrosion resistance among the inventive steels, evenbetter than reference materials No. Q9261 and Q9283.

The resistance to general corrosion was investigated by polarisationtesting in 0.05 M, H₂SO₄, pH=1.2, for the inventive steels No. 4 and 5and the commercial steels No. 1 and No. 3. The polarisation curves areshown in FIG. 7, and it is evident that the inventive steel No. 4 hadbetter resistance to general corrosion than the commercial steel No. 3and that the inventive steel No. 5 and commercial steel No. 3 hadapproximately the same resistance to general corrosion. Commercial steelNo. 1 had the best resistance to general corrosion among the testedsteels.

Manufacturing Process

In the process for producing a steel alloy for the manufacturing of aholder, holder detail for a plastic moulding tool or a moulding tool, aholder base, a holder detail base or a moulding tool base ismanufactured from a steel alloy with a chemical composition according tothe invention.

The steel of the invention is manufactured by producing a melt,preferably in an electrical arc furnace, an induction furnace or anyother furnace which uses scrap as the main raw material. Possibly, themelt is processed in a secondary ladle step to ensure appropriateconditioning of the steel before the casting process, i.e. alloying ofthe steel to target analysis, removal of deoxidation products etc. Thesteel does not need to be treated in a converter to lower the carboncontent further. The melt, having a chemical composition according tothe invention, is cast into large ingots. The melt may also be cast bycontinuous casting. It is also possible to cast electrodes of the moltenmetal and then remelting the electrodes through Electro-Slag-Remelting(ESR). It is also possible to manufacture ingots powder-metallurgicallythrough gas-atomization of the melt to produce a powder, which then iscompacted through a technique which may comprise hot isostatic pressing,so called HIPing, or, as an alternative, manufacture ingots throughsprayforming.

Said process further comprises the steps of hot working an ingot of saidsteel alloy at a temperature range of 1100-1300° C., preferably1240-1270° C., cooling said steel alloy, preferably in air, from the hotworking temperature to a temperature of 50-200° C., preferably 50-100°C., thereby obtaining a hardening of said steel alloy, followed bytempering twice during 2 hours at a temperature of 510-650° C.,preferably 540-620° C., thereby obtaining a tough hardened blank, andforming the holder base, the holder detail base or moulding tool base bymachining operation to a holder, a holder detail for a plastic mouldingtool or a moulding tool.

In an alternative process for producing a steel alloy for themanufacturing of a holder, a holder detail for a plastic moulding toolor a moulding tool, a holder base or a holder detail base or a mouldingtool base is manufactured from a ingot containing a steel alloyaccording to the above, said process comprising the steps of hot workingan ingot of said steel alloy at a temperature range of 1100-1300° C.,preferably 1240-1270° C. The hot working is followed by a cooling ofsaid steel alloy to an isothermal annealing temperature of 550-700° C.,preferably 600-700° C., where said alloy is subjected to an isothermalannealing at said isothermal annealing temperature during 5-10 h.Normally, the isothermal annealing is followed by a cooling of saidalloy to room temperature before the steel alloy is subjected to ahardening and tempering operation. The hardening is performed byaustenitizing the steel alloy at a temperature of 900-1100° C.,preferably 950-1025° C., and even more preferred at 1000° C., 30 min,and tempering twice during 2 hours at a temperature of 510-650° C.,preferably 540-620° C., thereby obtaining a tough hardened blank,thereafter forming the holder base, the holder detail base or themoulding tool base by machining operation to a holder, a holder detailfor a plastic moulding tool or a moulding tool. It is possible that thecooling from the isothermal annealing temperature to room temperaturecan be excluded, and that the heating to austenitizing temperature mayfollow directly after the isothermal annealing, but that has yet to beinvestigated.

1. A steel alloy comprising, in weight-%: 0.08-0.19 C, 0.16≦C+N≦0.28,0.1-1.5 Si, 0.1-2.0 Mn, 13.0-15.4 Cr, 0.01-1.8 Ni, 0.01-1.3 Mo, max. 0.7V, max. 0.25 S, max. 0.01 Ca, max. 0.0 O, in order to improve themachinability of the steel, and balance iron and unavoidable impurities,wherein the steel alloy has a microstructure which, in a tough hardenedcondition, comprises a martensitic matrix comprising up to 30 vol-%ferrite
 2. The steel alloy according to claim 1, wherein the steel alloycomprises 0.09<C≦0.15.
 3. (canceled)
 4. The steel alloy according toclaim 1, wherein the steel alloy comprises 0.05-0.20 N.
 5. The steelalloy according to claim 4, wherein the steel alloy comprises more than0.08 N and max 0.13 N. 6-7. (canceled)
 8. The steel alloy according toany of claim 1, wherein the total amount of C+N satisfies the condition0.17<C+N≦0.25.
 9. (canceled)
 10. The steel alloy according to claim 1,wherein the steel alloy comprises 0.6<Si≦1.2. 11-12. (canceled)
 13. Thesteel alloy according to claim 1, wherein the steel alloy comprises1.30-1.65 Mn. 14-17. (canceled)
 18. The steel alloy according to claim1, wherein the steel alloy comprises 13.5-14.5 Cr. 19-22. (canceled) 23.The steel alloy according to claim 1, wherein the steel alloy comprises0.20-0.50 Ni.
 24. (canceled)
 25. The steel alloy according to claim 1,wherein the steel alloy comprises 0.15-0.25 Mo.
 26. The steel alloyaccording to claim 1, wherein the steel alloy comprises 0.05-0.7 V.27-30. (canceled)
 31. The steel alloy according to claim 1, wherein thesteel alloy comprises 0.10-0.15 S.
 32. (canceled)
 33. The steel alloyaccording to claim 1, comprising: 0.09<C≦0.15, 0.08<N≦0.14, wherein0.17≦C+N≦0.25, 0.6-1.2 Si, 0.85-1.8 Mn, 13.5-14.8 Cr, 0.10-0.40 Mo,0.1-0.55 Ni, and 0.05<V≦0.20, wherein the steel alloy comprises amartensitic matrix, in a tough hardened condition, which contains up to15 vol-% ferrite.
 34. The steel alloy according to claim 1, comprising:0.10-0.15 C, 0.08<N≦0.13, wherein 0.8≦C+N≦0.25, 0.75-1.20 Si, 1.30-1.65Mn, 13.6-14.5 Cr, 0.15-0.25 Mo, 0.30-0.55 Ni, and 0.05<V<0.15, whereinthe steel alloy comprises a martensitic matrix, in a tough hardenedcondition, comprising up to 10 vol-% ferrite. 35-38. (canceled)
 39. Thesteel alloy according to claim 1, wherein the matrix comprises 0.05-6.5vol-% ferrite.
 40. A process for producing at least one selected from agroup comprising a holder base, a holder detail base, a molding toolbase for a holder, a holder detail for a plastic moulding tool, and amolding tool, the process comprising: manufacturing a steel alloycomprising, in weight-% 0.08-0.19 C, 0.16≦C+N≦0.28, 0.1-1.5 Si, 0.1-2.0Mn, 13.0-15.4 Cr, 0.01-1.8 Ni, 0.01-1.3 Mo, max. 0.7 V, max. 0.25 S,max. 0.01 Ca, max. 0.01 O, wherein the O, S, and Ca improve themachinability of the steel, and balance iron and unavoidable impurities,hot working an ingot of the steel alloy at a temperature range of1100-1300° C., cooling the steel alloy, thereby hardening the steelalloy, tempering the steel alloy twice for 2 hours at a temperature of510-650° C., thereby obtaining a tough hardened blank having amartensitic matrix containing up to 30 vol-% ferrite.
 41. A process forproducing at least one selected from a group comprising a holder base, aholder detail base, a molding tool base for a holder, a holder detailfor a plastic moulding tool, and a molding tool, the process comprising:manufacturing a steel alloy with a chemical composition according tocomprising, in weight-% 0.08-0.19 C. 0.16≦C+N≦0.28, 0.1-1.5 Si, 0.1-2.0Mn, 13.0-15.4 Cr, 0.01-1.8 Ni, 0.01-1.3 Mo, max. 0.7 V, max. 0.25 S,max. 0.01 Ca, max. 0.01 O, wherein the O, S, and Ca improve themachinability of the steel, and balance iron and unavoidable impurities,hot working an ingot of the steel alloy at a temperature range of1100-1300° C., cooling the steel alloy to an isothermal annealingtemperature of 550-700° C., and subjecting the steel alloy to theisothermal annealing at the isothermal annealing temperature for 5-10 h.42. The process of claim 40, wherein the steel alloy is produced, usingscrap as a raw material, by melting the scrap in a furnace. 43-47.(canceled)
 48. The process of claim 41, wherein the steel alloy isproduced, using scrap as a raw material, by melting the scrap in afurnace.
 49. The process of claim 41, further comprising: hardening thesteel alloy by austenitizing at a temperature of 900-1100° C. for 30min, and tempering the steel alloy twice for 2 hours at a temperature of510-650° C., thereby obtaining a tough hardened blank having amartensitic matrix containing up to 30 vol-% ferrite.